signal processing and filters for reg review ms ni
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Amplifier Function and Signal Filtering used in Sleep Disorders Technology
An Overview
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Amplifiers and Signal Filtering
You Need to know: What differential amplifier is What CMR is and how it works How TC filters work and TC relationship to LFF How LFF and HFFs work to reduce unwanted
frequencies How a 60Hz notch filter works Know basic polarity problems The difference between an AC and DC amplifier The difference between gain and sensitivity
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Terms for Basic ElectricityImpedance: The total opposition to current flow including resistance, inductance and capacitance. Measured in ohms.Resistance:The opposition to the flow of current. Measured in ohms.Volt: Measurement of the force pushing current (electrons) through a conductor. Polarity: Refers to the positive or negative poles of a device
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Alternating Current (AC)
Alternatively flows between positive and negative (protons+ and electrons-)Alternates at 1/60 sec in USA 1/50 sec in Europe
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Ohms Law
Voltage is equal to current flowing in the circuit, multiplied by the resistance.
Voltage = E (volts)Current = I (amps)
Resistance = R (ohms)
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Voltage
Resistance
E
R
V = I x R
I ER
R EI
Current
I
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The Polygraphic Circuit
The function of the polygraph is to transform voltages generated by body into an interpretable record
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Chart Drive
- --- -- +
++
++
+
G1G2
Paper / computer
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Signal Processing
Electro conductive materialElectrode These act as a small capacitor
Wire to head boxExploring electrode C3, C4 O1, O2
Reference electrode A1, A2
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Ground
Leakage current results when there is either a break or a shunt in the circuit.Patient grounding is essential in artifact elimination and proper amplifier function.
Symbol for Ground
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Differential Amplifier
Form of AC amplifierAmplifies the difference between input (grid) 1 and input (grid) 2Rejects any similar information between G1 and G2 (CMR)
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Common Mode Rejection
Common function of differential amplifierRejection of similar activity between input 1 and input 2 Identical information rejected at near 100%
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Polarity and Localization
Input 1 When input one is negative, the pen goes
up, when input one is positive, the pen goes down.
Input 2 When input two is positive, the pen goes
up, when input two is negative, the pen goes down.
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Basic Polarity and Localization
NegativePositive
PositiveNegativeInput 1 (G1)
Input 2 (G2)
Input 1 rules
Input 2 rules
Up rules Down rules
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Common Mode Rejection Ratio
Common Input Signal Output Signal
Should be high ie. 100,000 to 1
AC coupled Amplifier
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Polarity Calculation
26 uv
-15 uv
26
- 15
+ 41 uv
+ 41 uv
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Galvanometer
Limited to a maximum response of 90 - 120hz
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Amplifier/Machine Calibration
All filters set the same
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Montage Calibration
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Time Axis
The point (time) at which each pen falls along the same recording line
The galvonometer or pen mount may be adjusted to correct problems
Not a problem with digital systems
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Damping
Adjustment to reduce overshootReduces erroneous fast-wave artifactOnly adjusted on polygraphs Not digital systems
good
Under damping
Over damping
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Mechanical Baseline
Pen spacing withoutamplifier input to
pens.
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Electrical Baseline
The “electrical zero” of a recording pen position
Found by turning the individual chart amplifier power off and on while adjusting the baseline knob.
EZ if found when the pen stops changing positions between power-on, and power-off
Electrical Zero
Mechanical Baseline needs adjustment
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1. Unequal sensitivity
2. Pen alignment—time axis
3. Pen baseline off
4. LFF different setting
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Gain vs. Sensitivity
Gain is a measure of the ability to change the magnitude of the input voltageSensitivity is the amount of output (pen deflection) to a given input voltage
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Voltage
Pen DeflectionSensitivity
S
V
PD
V = S x PD
S VPD
PD VS
Sensitivity
S
uV
uV/mmmm
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Sensitivity Calculations
Sensitivity is at 5uv/mm the voltage is 50 uv what is the deflection?
D = V/S
D = 5uv/mm 50uv
D = 10 mm
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Sensitivity is set at 7 uv/mm and the deflection is 12 mm. What is the voltage?V = S X D
V = 7uv/mm X 11 mm
V = 77 uv
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Voltage is 75uv and the deflection is 15 mm. What is the Sensitivity setting?
S = V/D
S = 15mm 75 uv
S = 5 uv/mm
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60 Hertz Notch Filter
Also known as an AC FilterIn North America, all AC electrical current oscillates at 60 Hz.Uses CMR by tapping into line current and measuring it against output.
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0
10
2030
40
50
60
7080
90
100
40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 76 78 80
60 Hertz Notch Filter
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Units of Measure Used in Filtering
Hertz (Hz) Most common unit in electroneurodiagnostics
Second (sec) Used in Time Constant (TC) filter calculations
Decibels (dbl) Most basic unit of measure in frequency filtering Rarely referred to in sleep diagnostics
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Time Constant (TC)
Units = seconds (s)The time it takes in seconds for a waveform to drop to
37% of its calculated amplitude
Faster TC – Reduces amp. of slow frequencies
Low cut – high pass filter
100%
37%
Time (s).3.003 1
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TC (sec) and LFF (Hz) Relationship
TC and LFF work with same goal Reduction of slow frequencies Different units of measure to describe the same
action
Conversions:
1
2π x LFF TC =
1
2π x TCLFF =
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Basics of LFF and HFFs
Reduces amplitude of listed frequency at a fixed value 20% (Nihon Kohden) 30% (Grass)
Reduces amplitude of frequencies above (HFF), or below (LFF) at linear levels
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Low Frequency Filters
Designed to reduce the amplitude of frequencies at and below the selected frequency “Low cut, high pass filters”.
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Expected Amplitude Decay: LFF = 10HzBased on Nihon Kohden filters
Reduces amplitude of a 10 Hz signal by 20% Reduces a 5 Hz signal by 50%
Reduces a 2.5Hz signal by 75% Reduces a 1.25 Hz signal by 87.5%
0
10
20
30
40
50
60
70
80
90
100
Am
pli
tud
e
1.25Hz 2.5Hz 5Hz 10Hz
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Low Frequency Response Curve
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High Frequency Filters
Designed to reduce the amplitude of frequencies at and above the selected frequencyHigh cut – low pass filters
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Reduces amplitude of a 70Hz signal by 20% Reduces a 140 Hz signal by 50%
Reduces a 280 Hz signal by 75% Reduces a 560 Hz signal by 87.5%
Expected Amplitude Decay: HFF = 70Hz(based on Nihon Kohden filters)
0
10
20
30
40
50
60
70
80
90
100
Am
pli
tud
e
70Hz 140Hz 280Hz 560Hz
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High Frequency Response Curve
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Filters Effect Amplitude & Phase
Note: The Higher the filter the lower the amplitude
Digital Filters will not cause a Phase Shift
High Filter
Low Filter
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Direct Current Amplifiers
Used to amplify signals which represent either all negative, or all positive voltage Not alternating between – and + (AC)Used most often with slow trending data (SaO2 and heart rate trending)No LFF or HFF filters utilized TC infinite (No frequencies to filter)
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Wheatstone Bridge Circuit
A four-arm bridge used to measure resistance in a circuit Commonly used in DC amplifiers when resistance devices are in use Strain gauges
NPT Effort gauges
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Analog to Digital Conversion
Brainwaves and other PSG signals are analog or continuousDigital is based on values of 0 or 1ADC samples the signal and assigns a digital value
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Sampling Rate
The rate at which the computer program samples the analog information to convert it to digital signal. The faster the sampling rate the more
accurate the tracing is The faster the sampling rate the larger the
patient data file will be
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Sampling Rate
Nyquist principle The sampling rate must be at least twice
as fast as the fastest frequency that will be recorded.
ASET Standard is three times faster than the HFF setting
Can vary by channel 200+/sec for EEG 10/sec for respiratory
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Sampling Rate
Aliasing Sampling rate too low
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